security through obscurity is generally not about keeping a well known encryption scheme's keys "private". It's generally about not knowing how a system works

security through obscurity is generally not about keeping a well known encryption scheme's keys "private". It's generally about not knowing how a system works at all. In this case (and the same for blurays actually), we know exactly how the system works, given the keys we could decrypt the content, but the keys are kept "well" protected (depending on widevine level, different levels of protection). In BluRay land enough player keys have leaked to make it basically irrelevant. It's also harder to determine whose keys are being used to decrypt, making it harder to revoke. In an online widevine world where one has to use one's baked in device keys to get the content key, its much easier to determine if a single device's key is being used an abnormal amount of time and then revoke it. while one can view the efforts to protect a widevine l3 key as security through obscurity, its mostly there to make the effort hard enough that most people are interested in doing it, than to keep it perfectly secure.